Replacement*Bridges*for*LowTraffic*Volume*Roads.*Part*One ... · Replacement*Bridges*for*LowTraffic*Volume*Roads.*Part*One:* Concrete*Hollowcore*Bridges*!...

Replacement Bridges for Low Traffic Volume Roads. Part One: Concrete Hollowcore Bridges Chris Dowding, Director – Structures Group, TOD Consulting -­ engineers and project managers November 2015 Abstract Australia travels and moves its freight over an estimated 20,000 Timber Bridges. Local Governments, National Park Authorities and Timber Forestry Operators manage roughly 14,000 of these structures. Budget-­stretched Asset Managers have choices to make: maintain or replace? Costs for either strategy have leapt over the last two decades, with limited hardwood availability, and changes to Safety, Environmental and Labour legislation. Ultimately, logistics companies are making daily requests for overload permits: vehicle loads have increased beyond the safe capacity of existing timber bridges. Engineers have responded to these changes with very robust standards and solutions that are entirely appropriate for busy roads, motorways and highways. But are these solutions appropriate for quiet roads “in the bush”, loaded by a few trucks each day? We look at bridges constructed with precast Hollowcore planks. Manufacturers produce these 1200mm planks with great efficiency, in thicknesses up to 420mm. When combined with a structural concrete slab cast over the top, shear reinforcement in the plank gaps, and transverse bending reinforcement in the slab, the composite structure can be designed to carry SM1600 loads. Two robust, single span (12m) bridges were built in 2015, over sensitive waterways, for costs competitive with large box culverts. Keywords: Timber bridge replacement, Fish-­passage, Alternative to box culverts

Introduction Bridges link people, services and goods together. So do planes and cars. So do mobile phones. Each are a device that assists society to connect. Like any device, a bridge can become superceded by changing needs and expectations. We want to upgrade. We particularly want to upgrade Australia’s hardwood girder bridges, because these are dilapidating rapidly. Hardwood girder bridges were built throughout the late 19th and 20th century, with new construction ending around 1970. These bridges got Australia moving, and were a

very innovative solution to use the resources available in a young country with very limited resources. It is estimated that around 20,000 hardwood girder road bridges remain in service around the country, although this number is difficult to confirm, because Australia doesn’t have a publically-­available national bridge register, in contrast to the US, which does.

We estimate an average replacement cost of $1,000,000 per bridge with associated roadworks, using conventional precast concrete planks, connected by transversely post-­stressing or by cast-­insitu topping slab. For 20,000 bridges, this totals to a replacement cost of $20 billion. A large majority of in-­service hardwood girder road bridges are managed by very budget-­constrained Local Governments, who must rely on competitive grants from the State or Federal Governments to fund bridge replacements. Attempts to simply impose speed reductions and load limits on structurally deficient bridges are, for the most part, poorly received. But these problems aren’t unique to Australia: other developed nations, including the UK and the US, also have budget constraints which requires low cost solutions to maintain safety. The community at-­large doesn’t understand the funding difficulties, and believes that bridges should just work. What happens when a bridge no longer works – when the link is broken? The sociological effect of a failed bridge In 1975, a ship (the Lake Illawarra) struck one of the central piers of the Tasman Bridge, in Hobart. Three spans collapsed and 12 people died.

(Photo credit: http://www.abc.net.au/news/2015-­‐01-­‐05/tasman-­‐bridge-­‐disaster-­‐40th-­‐

anniversary/5998396)

This bridge had connected the west and east sides of the city. At that time, the east was essentially a residential area. The majority of important services and facilities, such as hospitals, workplaces and schools, were on the more-­developed west side. East-­side residents faced a 1.5 hour drive to the city, instead of 3 minutes, and police records show that crime rates soared on the east side of the city. It is said that ‘the physical isolation led to setting aside of bonds of normal social life.’ Although bridge repairs were completed three years later, the disaster remains deep in the psyche of Hobart residents to this day. Essentially, these issues could also occur for smaller populations on each side of a dilapidated timber bridge, which becomes a national economic and sociological risk when multiplied by 20,000 bridges. Two sites with collapsed timber bridges

In January 2013, two 10m span timber bridges were washed away during a major flood event. These were located at Neilson Rd, Kandanga Creek and Elliott Road, Cedar Pocket;; within the Mary River catchment, in the southeast of

Queensland, Australia.

As a brief background, in 2006, the State Government had commenced plans to dam the Mary River. The Government’s forced resumption of freehold farmland and non-­consultative approach led to community discord, displacement and anger. When the dam was cancelled in 2009 by the overriding Federal Government decision, the community was left morally vindicated, yet socially decimated. Gympie Regional Council asked my company to provide them with replacement options:

§ Both bridges were on roads with very low traffic volumes: closure was considered as an option for both sites, due to alternative routes

§ The environments of both sites are a mix of dairy agriculture and nature § Flood flow velocities of both sites are rapid, at 3.4 to 3.9 metres per second § Council’s initial preference was for box culverts, due to perceived low

construction cost

§ The community’s preference was for a solution that caused no environmental harm to the river catchment, which made box culverts a less desirable option

Structural and Environmental factors Our initial review of the sites indicated two major issues:-­ high risk of scour due to rapid velocities, and both creeks were mapped as important habitat for fish. In our opinion, box culverts could be undermined or destabilised by floodwaters, or in the worst case, overturned. The importance of Fish habitat meant that any adopted solution would need to make allowance for fish passage. Fish passage is a relatively new consideration for waterway crossing design in Australia. In the US, it has been a design requirement for decades. Put simply, fish travel upstream, for breeding and other purposes. Waterway crossings in fish habitats need to facilitate this upstream movement. Historically, pipe culverts and box culverts have not facilitated upstream fish movements. The downstream apron of these structures can become scoured so deeply that fish can’t make a leap upstream. Furthermore, the flow depth is often too shallow and too fast to allow fish to swim through the structure. A good example of fish being unable to make it through a culvert structure, and the subsequent fish passage restoration, can be seen at: http://youtu.be/0ZSex8ofXx4

(Photo credit: http://www.fsl.orst.edu)

For box culverts, the current solution is to use larger boxes and excavate the base deeper so it is founded below the natural creek bed level. This tends to slow or eliminate the downstream scour, but digging deeper into the creek has significant consequences to the bed of the waterway, which

can be seen in the above photograph. (Photo credit / source: www.southernenvironmental.org) These consequences are often seen as temporary, but for threatened species, the repercussions may be longer term, and it would certainly be desirable to avoid them, if possible. Can we build a Bridge for Large Box Culvert Costs? Our goal was to achieve the best fish passage for the dollars spent. We carried out preliminary designs and considered four alternatives for each site, which can be seen in Table 1.

Table 1: Cost, environmental impact and flood resistance comparison of different crossing options Both bridge options had the advantages of:

§ better flood resilience, due to tiedown and scour protection measures § better provision for fish passage, because of no disturbance to the creek bed

The Hollowcore bridge was an option we hadn’t designed before. There are some Hollowcore road-­bridges in Victoria, and one in Brisbane, but to the best of our knowledge, these hadn’t been designed to be overtopped by fast-­moving floodwaters regularly. We incorporated flood-­resilience measures into the preliminary estimate, and we further applied a 35% contingency, as compared to 10% for the more conventional options. The Hollowcore option stood out as the overall lowest cost option, and Gympie Regional Council asked for it to be progressed to detailed design.

Hollowcore concrete planks Hollowcore concrete planks are precast planks with pre-­tensioned strands as the reinforcement. In Australia, they are manufactured in Melbourne and Brisbane, to thicknesses between 100mm to 420mm. In Europe, Canada and the US, these are produced in thicknesses up to 500mm. Hollowcore planks are extruded between two continuous steel sideforms, rather than being cast in fixed metal forms. The concrete is vibrated and compacted by the extrusion machine. The process is very efficient, with only two labourers required, as compared to several for conventional precast bridge planks in metal formwork. The concrete for planks is a dry mix, commonly with a characteristic compressive strength of 50MPa. Cast-­insitu slabs are normally placed over the top of the planks, which act compositely with the planks to give a deeper structural depth with greater bending moment capacity. Importantly for structural engineers, there are no shear ligatures in Hollowcore planks, as no practical way has been found to incorporate these into the extrusion process. Furthermore, for Hollowcore planks over 320mm thick, several papers have demonstrated that shear strength is lower than predicted by conventional ACI and Eurocode formulas, and recent practice is to reduce shear capacity by 50%. Even so, the compressive stress induced in the concrete by the pre-­tensioned strands creates significant shear capacity. Typically, Hollowcore planks are used for building construction, with spans up to 18 metres being very suitable for multi-­storey carparks. As noted in the previous section, there are several existing Hollowcore roadbridges in Australia. Our goal was to develop a system to ensure flood-­resilience for the new Hollowcore bridges at Neilson Rd and Elliott Rd.

Designing a Hollowcore Bridge A detail of the composite (Hollowcore planks and insitu slab) deck can be seen in the below diagram.

Figure 1: Hollowcore single lane deck arrangement with topping slab We addressed each design consideration as follows: 1. Durability: The two sites for our project were classified as exposure classification “B1” in accordance with AS5100.5. Standard cover to the strands in Hollowcore planks is 35mm, which is sufficient for exposure classification up to B1 for a precast 50MPa element, per Table 4.10.3(A) of AS5100.5. Cover can be increased to 45mm relatively easily, so theoretically the planks could theoretically be designed to suit a site with “B2” exposure, but not for exposure “C”. If we were designing for a B2 site, we would specify additional protective measures, such as silane coatings and carefully designed blended-­cement concrete. But the reader should bear in mind that we consider these bridges to be suitable for low traffic volume roads, meaning low population areas. In Australia, these are generally found more than 1 kilometre inland, meaning the majority of suitable sites for Hollowcore bridges will be exposure classification B1 or lower.

2. Fatigue: Both sites experience less than 50 vehicles per day, with around 5 of

these being heavy commercial vehicles. We allowed for 100 vehicles per day in our design, with 10 of these as commercial vehicles.

Clause 2.5 of AS5100.5 requires fatigue of reinforcement and strand to be considered. If the effective number of stress cycles is less than 500,000, no further consideration is required. Using clause 6.9 of AS5100.2, we calculated that 500,000 cycles equated to 25 heavy vehicles per day. As our design traffic volume (10 heavy vehicles per day) was less than 25, there was no need to consider fatigue further.

3. Transverse shear capacity of deck: When calculating transverse shear capacity, we set the structural depth as the depth of the Hollowcore Planks. We conservatively excluded the benefit of the additional depth provided by the topping slab. (The conventional practice is to set the structural depth for shear calculations as the combined composite depth of the Hollowcore planks and the topping slab).

We adopted the (“beam-­type shear”) formula of phi.Vuc, as given in Clause 8.2.7.2 of AS5100.5 – 2004, and applied a 50% reduction factor to be consistent with “beams with no shear reinforcement”, as given in Clause 8.2.5 of AS3600 – 2009. Shear force V* was less than the adopted capacity of 0.5 x Phi.Vuc. This adopted capacity (0.5 x phi.Vuc) was around 40% of the (US-­based)) capacity values given by the PCI Manual for the Design of Hollow Core Slabs (6), even after the web cracking capacity (phi.Vcw) was reduced by a 0.5 modification factor to allow for the findings of Hawkins & Ghosh (7). (Hawkins & Ghosh (7) found that web cracking dominates over flexural shear failure for Hollowcore planks exceeding 320mm thickness, and subsequent practice in the US has been to reduce the predicted value of Vcw by 50%) Minimum shear steel, in the form of L-­shaped mesh, was provided in the key joints and outer cores, as required by Clause 8.2.5 (a) of AS5100.5 -­2004, although arguably the shear capacity would be sufficient without this steel, given the higher capacities indicated by the PCI calculations. The L-­shaped mesh provides two important additional benefits: a) The tiedown effect of the mesh ensures that the underside of the topping

slab remains in contact with the Hollowcore planks during the vibratory effects of passing trucks. This in turn ensures that composite action is maintained.

b) The mesh tiedown effect prevents flood-­induced liftoff and overturning of the topping slab.

4. Bending capacity: Calculation of ultimate bending capacity assumed that the Hollowcore planks and topping slab act compositely together as one section. The composite action relies on the cohesion of the topping slab against the top surface of the Hollowcore Planks:

• As noted in 1) above, the L-­shaped mesh in the key joints and outer cores, performs an essential role in tying down the slab, by ensuring that the underside of the topping slab remains in contact with the Hollowcore planks during the vibratory effects of passing trucks. We conservatively excluded the mesh from the calculation for composite action (see longitudinal shear capacity, Clause 8.4, AS5100.5-­2004).

• T44 design load – sufficient cohesion is achieved with the standard longitudinal broomed finish to the Hollowcore planks.

• SM1600 design load – requires a non-­standard transversely broomed finish to the Hollowcore planks, to achieve the required cohesion

The number of bottom strands were increased to 16 No. x 12.7mm diameter. These were pre-­tensioned to 110.4kN. The top strands were increased to 4 x 9.3mm diameter, pre-­tensioned to 40.8kN each. Total pre-­tension was kept below 2000kN, which was the limiting capacity of the Hollowcore stressing head in the precasting yard.

Bending capacity was calculated in accordance with the principles of Clause 8.1 of AS5100.5.

5. Topping slab: The topping slab was designed and reinforced to span 1200mm under a W7 wheel load (70kN x dynamic load factor x ultimate load factor). The intention was to span across the thin flanges of each Hollowcore plank, and transfer the stresses into the internal webs, key joints and outer cores of the planks. The topping slab also transfers load transversely to the adjacent plank, in conjunction with the concrete-­filled key joint, so that the wheel loads on one side of a vehicle are supported by two planks. The key joint was concrete-­filled to approximately 80% of the plank depth, with S40/10 concrete (40MPa compressive strength, 10mm aggregate) and the slab was poured with the same concrete, in the same pour operation as the key joints and outer cores. The load-­transfer function of the key joint has been well-­tested in New Zealand, where Hollowcore plank bridges (spans up to 18 metres) were installed with a shallow key joint (depth < 50% of plank depth), and no topping slab, for many decades. A maintenance issue was observed in the asphalt deck wearing surface of longer spans: longitudinal cracks appeared in the asphalt above the key joints. The current solution is to increase the depth of the key joint, and provide either transverse prestress or a structural overlay slab composite with the deck units. We elected to provide a structural overlay slab (topping slab) for our project, because it also acts as the deck wearing surface, with no requirement for asphalt overlays.

6. Foundations/Footings: These are discussed in the next section

Constructing a Hollowcore Bridge Our aim in developing a Hollowcore solution for the two bridge sites, was to provide a solution that was a similar cost to large box culverts. Accordingly, the level of construction effort and required expertise was considered carefully. Most Local Government have their own construction crews: a) Local Government construction crews are normally very familiar with box culvert construction.

b) They are not always familiar with Bridge construction

We considered the areas where specialist skills and equipment are required for bridge construction, and where possible, we simplified the task.

Piling AS5100.2 requires bridges to resist Q2000 flows without collapse, although scour damage is excepted. The lowest-­risk industry solution is to found the bridge on deep piles, which extend well below the anticipated scour depth. Deep piles normally require a large piling rig, with a specialist crew. Establishment and site costs are significant, and there are usually additional costs, like providing a robust gravel hardstands (up to 1 metre thick) at each pile location for the rig to sit on. After the piling rig has left, scour protection is usually installed around the abutments and sometimes around piers. This is a simpler operation with an excavator. Generally, scour protection is sized for flood velocities somewhere between Q20 and Q100 event. The scour protection can be a variety of materials, but the preference seems to be headed towards self-­healing systems, such as un-­grouted riprap or gabion baskets. In contrast to bridges, large box culverts are normally founded in a creek bed on shallow footings without piles:

§ Potentially, the same flood forces and issues that apply to bridge design, could also apply to box culvert design: scour that undermines shallow foundations, log impact, debris mats, buoyancy, drag and lift forces.

§ Box culvert designers do not use piles to address these issues. The industry seems to rely instead on rule-­of-­thumb scour protection measures, such as riprap, grouted rock, rock-­filled wire mattresses, or shotcrete. Tiedown, if there is any, is limited to the use of angle brackets to connect the precast culvert cells to the shallow footings.

From the writer’s viewpoint, it appears that flood-­related design methods for box culvers and bridges have evolved down different paths because of differing design viewpoints and because of a generalised perception of risk:

§ Box culvert designers tend to be focused more on managing water flows, and may be less aware of the worst-­case flood forces that could damage their structure

§ Bridge designers tend to be more focused on worst-­case forces, and may be less aware of how to manage the flow of water under and around their structure.

§ Traditionally, box culverts cost less to construct per square metre than bridges. If a box culvert fails, the replacement cost is “low”. If a bridge fails, the replacement cost is “high”.

In reality, the risk profile of both box culverts and bridges change depending on the circumstances. The comparison of risk must change if:

a) the cost of a bridge could be lower than the cost of box culverts b) the cost of box culverts has increased due to fish passage requirements c) the road is a secondary road, particularly if there are alternate routes

d) the bridge can span clear from bank to bank without any central piers, in contrast to box culverts founded in the creek bed

e) the banks are protected with scour protection sized for Q2000 flood velocities

f) the bridge is tied down at each bank

For our project, we increased the size of the ungrouted riprap to suit a Q2000 flood event. We provided micropiles to carry both compression and tension loads, and these can be installed with a small rig for relatively low establishment costs.

Topping slab

Reinforcement of the topping slab was detailed in mesh, to make the topping slab construction as similar to slab-­on-­ground construction as possible. Side forms were constructed with timber propping into the creeks, but could have alternatively been fixed to the sides of the Hollowcore planks using drill-­in threaded concrete fasteners. Load test We carried out Static and dynamic load tests of both bridges with a 22 tonne dump truck (dual rear axle), and also tested Elliott Rd bridge with a 50.2 tonne truck and dog trailer. Static deflection under both trucks was 2 millimetres, which was less than our theoretical prediction of 4 millimetres. The similar deflection for both trucks was due to the limited number of axles that could be on each 12m bridge. The static deflection results proved the theory that Hollowcore planks act compositely with topping slabs. We do emphasise the importance of the L-­shaped mesh between the plank key joints and the topping slab for bridge applications. This mesh ensures that the

topping slab and the Hollowcore planks remain in contact during truck-­induced vibration and bounce. Construction cost At Practical completion, the final construction cost was $234,554 average per bridge: § < $261,131 pre-­construction estimate including 35% contingency § < $301,932 estimate for “Fish-­passage-­ready” large box culverts § < $371,932 estimate for Conventional precast plank bridge § All figures excluding GST & Roadworks. Conclusions Neilson Rd & Elliott Rd Bridges were single lane bridges, designed for T44 load (12 metres single span), in a very-­low-­traffic, low-­risk rural environment. These bridges recently won an award with the Institute of Public Works Australia – Queensland Division (IPWEAQ). With the learnings from this project, we have been able to design two larger Hollowcore bridges for SM1600 loading. These will be two-­lane, 11 and 13 metre span bridges to suit a rural-­residential environment along Fleming Rd, Gympie, Queensland. These will be constructed during 2016. In conclusion, we see Hollowcore bridges having a potential role in the options between box culverts and standard precast bridge solutions, for the following applications:

§ Country roads (low traffic volume < approximately 250 vehicles per lane per day, including 10% Commercial vehicles)

§ High-­environmental-­value waterways – creek bed kept in natural state § 100-­year Design Life in B1 exposure environments

For more information:

§ Video: https://youtu.be/G3UtvDOXJAQ § www.todconsulting.com

References 1. https://en.m.wikipedia.org/wiki/Tasman_Bridge_disaster

2. ABC News (Australia). (2015, January 5). Tasman Bridge disaster: 40 years on, Tasmania remembers night of deadly accident. Retrieved from http://mobile.abc.net.au/news/2015-­01-­05/tasman-­bridge-­disaster-­40th-­anniversary/5998396

3. Tasmanian Archive and Heritage Office., (2014, December). Tasman Bridge

Reconstruction [Video file], Retrieved from http://youtu.be/hJZIN2xRN2s 4. https://en.m.wikipedia.org/wiki/Traveston_Crossing_Dam

5. Dunklin, T.B., and Larson, Z., (2012, December). Cedar Creek Fish Passage Restoration [Video file], Retrieved from http://youtu.be/0ZSex8ofXx4

6. Buettner, D.R., and Becker, R.J., Manual for the Design of Hollow Core Slabs, Precast/Prestressed Concrete Institute, 2nd Edition, Chicago, IL, 1998

7. Hawkins, N.M., and Ghosh, S.K., “Shear Strength of Hollow-­Core Slabs”, PCI Journal, v. 51, January-­February 2006, pp. 110-­114

8. Simasathien, S. and Chao, S., “Shear Strength of steel-­fiber-­reinforced deep hollow-­core slabs”, PCI Journal, July-­August 2015, pp. 85-­101

9. Gray, A., et al, New Standard Precast Concrete Bridge Beams Stage 1 – Research & Identify Proposed Standard Beam Shapes and Spans, Transfund New Zealand, Wellington, NZ, 2003